The following relates to the water heater arts, water heater control arts, and related arts.
In electrical power systems, the power generation should match the power draw in order to maintain desired grid operating characteristics such as voltage, frequency, and so forth. Conventionally, this is achieved by adjusting the power generation to meet the demand, for example by bringing ancillary diesel power generators online or offline. Energy can also be stored in batteries, flywheels or the like, but this adds infrastructure expense and power losses.
Another approach for balancing generation and load is demand response, in which the load draw is increased or decreased to improve match with power generation. Various demand response techniques exist, such as peak shaving or load shifting, which operate on relatively long time scales typically on the order of tens of minutes to hours. Another demand response technique, called frequency regulation, increases or decreases load draw rapidly in response to an automatic generation control (AGC) signal associated with the electrical power grid and updated, for example, every four seconds. Frequency regulation operates at shorter time scales on the order of seconds to tens of seconds.
Water heaters are an attractive type of load for use in demand response systems, because a water heater stores thermal energy as hot water. A demand response system can remotely control the water heater in order to store or extract energy on demand, and relatively quickly. In executing remote control of the water heater, the demand response system should operate in a manner that ensures the water temperature is maintained within a temperature range that is acceptable to the end user. Additionally, the remote control should have safeguards to ensure that a failure of the remote control cannot cause the water to overheat to a dangerously high temperature capable of harming a person using the hot water output.
In one conventional approach, the remote control is implemented as a retrofitted power relay capable of opening to switch off electrical power to the water heater. When the relay is closed, electrical power is delivered, and the water heater is controlled in the usual way by its thermostat(s) in accord with the temperature set point adjusted by the end-user. This approach is suitable for demand response functions such as peak shaving, as it can operate to prevent the water heater from drawing power during power curtailment time intervals. It has numerous advantages: the remote control cannot cause unsafe water overheating, installation is simple, and the existing thermostat settings and control remain in effect so that the remote control is transparent to the end user (except possibly during curtailment intervals). However, this approach cannot be used for demand response modes that require actively energizing the heating elements, such as load shifting or frequency regulation.
In order to retrofit a water heater to enable remotely turning the heating element(s) both on and off, the existing thermostat may be replaced. However, this complicates the retrofit process and adds cost. Additionally, the remotely controllable replacement thermostat must be robust against failure so that the remote control cannot inadvertently raise the water to an unsafe temperature.
In a variant approach, the existing thermostat is not removed or deactivated, but rather its set point is raised to a high temperature so that it is always keeping the heating elements on. The added remotely controllable thermostat then performs the actual control. Since the original thermostat set point is raised to a high value, the replacement thermostat still must be robust against failure so that the remote control cannot inadvertently raise the water to an unsafe temperature. One way to do this is to add a mechanical mixing valve to add cold water at the hot water outlet to limit the maximum outlet temperature. This again increases retrofit complexity and cost.
In either of these approaches, the original temperature settings of the existing water heater are lost, causing a change in the delivery temperature of hot water. Additionally, the end-user must perform any temperature set point adjustments using the replacement temperature control system, rather than by using the familiar existing temperature set point adjustment of the water heater.